Preface

Introduction: Maps or Globes

To introduce the topic of this special issue, it might be helpful to focus initially on the context of discrete global grid systems (DGGSs) and some of the fundamental issues that surround their development, adoption, and use. We have known for centuries that we live on a curved surface, and we have found numerous ways of dealing with the complexities that curvature causes in mapping and related activities. If we can imagine removing the vegetation, buildings, and water to leave a "bare earth," we are left with a very complex surface that in some respects resembles a fractal. When the comparatively smooth isopotential surface represented by mean sea level is extended in the imagination under (and sometimes over) the land, the resulting geoid has bumps and hollows that suggest the metaphor of a potato. The science of geodesy has taken simplification one step further by approximating the potato as an ellipsoid using the Earth's axis of rotation, allowing us to ignore the bumps and hollows and to define and measure latitude. Here the metaphor of an orange becomes helpful, though the bulging of an orange is along the axis rather than perpendicular to it (echoing the disagreements about the shape of the Earth between the British Newtonians and the French Cartesians of the early eighteenth century). And in a final act of simplification we often think of the Earth as approximated by a sphere, just as π is sometimes approximated as 22/7.

Yet from our earliest years, much of our human experience ignores the curvature in favour of a flattened Earth, mostly with north at the top and Europe and Africa in the centre. We are used to the gross exaggeration of the size of Greenland that is an effect of the Mercator; to Australians who think of themselves as "down under;" to globes that are mounted with the axis almost vertical and the North Pole at the top; and to the phrase "Sub-Saharan Africa," which is less than or below Saharan Africa only in the sense of its north latitude. We talk irrationally of flying from Seattle "down" to Los Angeles but "up" to Vancouver.

There are many arguments in favour of flattening, of course. The pre-digital need to print, store, and distribute geographic information clearly favoured the map rather than the globe; the need to see all of the Earth at once must always require some form of projection; appropriately chosen projections allow certain functions to be simplified, such as tracing a course at a constant bearing using a Mercator projection; and the analysis of data in a flat, two-dimensional world is in general faster and simpler than analysis on a curved surface. Moreover, the human eye creates a two-dimensional image; it is only through the power of stereographic vision, and the ability to create "structure from motion," that the mind is able to build a three-dimensional representation of what is seen.

But while the digital world has opened many new possibilities, mapping practices have clearly left legacies in the ways we talk and think about the geographic world. We still present geographic information systems (GIS) as containers of digital maps rather than digital globes (Goodchild 2018), and we still make simple mistakes when we forget the distortions that occur in projection (National Research Council 2006). Many airlines present flattened views of the Earth in their cabin flight trackers, confusing passengers who think that the shortest path between two points should be a straight line.

Digital Earth

Implementation of a digital globe had to wait for the development of suitably powerful display systems, and it was not until the early 1990s that such systems became affordable. Development followed rapidly. The first public demonstrations of a representation of the Earth being panned and zoomed on a computer screen date from SIGGRAPH 1994, by which time it had become possible to warp a two-dimensional image of the Earth onto an imaginary sphere using readily available routines. By 2005, sufficiently powerful graphics accelerators had become standard in PCs, allowing Google to release Google Earth, based on software originally developed by Keyhole. Yet even today, Google Earth...